Method of cooking sulphite pulp



Feb. 25, 1958 c. F. ROSENBLAD METHOD OF cooxmc SULPHITE PULP Filed July 6, 1955 ATTORNEY 7 METHOD OF coomo SULPHITE PULP Curt Fredrik Rosenblad, Princeton, N. 1., assignor to Aktiebolaget Rosenhlads Patenter, Stockholm, Sweden, a corporation of Sweden Application July 6, 1955, Serial No. 520,223

Claims priority, application Sweden July 6, 1954 3 Claims. (Cl. 92-11) In cooking or digesting sulphite pulp the digesters are usually charged with digestion acid from a storage tank in which digestion acid enriched in S is kept at a temperature, for instance 7080 C., at which it is desired to supply the digestion acid to the digester and at a pressure of about 3 atm. overpressure required for the SO -gas to be kept dissolved in the digestion acid at this temperature.

The storage tank must consequently be constructed for a rather considerable overpressure and also be well insulated for avoiding large losses of heat. Due to this fact and because the storage tank should have a great volume, it follows that said tank will be expensive. The digestion acid is usually heated in indirect heat exchangers before being supplied to the storage tank, or during circulation through same. Usually this process takes place during a considerably longer period of time than the time during which the digestion acid is transferred from the storage tank to the digester, so that the heat exchangers required for this purpose are maintained more or less in continuous action. It is true that by doing so one gains the advantage of being able to dimension the heat exchangers for a flow per time unit which is considerably lower than the quantity of digestion acid per the same unit of time while being supplied to the digester but, on the other hand, the serious drawback from a service point of View then arises that the heat exchangers cannot be washed out without interrupting the output.

The present invention relates to a new method of charging the digester with heated digestion acid and has the object, among others, of obviating the need of storing hot digestion acid under pressure, as well as rendering it possible to clean out periodically the indirect heat exchangers which serve for heating the digestion acid without disturbing operations.

A further object of the invention is to enable carrying out the heating of the digestion acid in a most expedient manner, both as regards economy in heat and service technique. It is a special prerequisite of the invention that the digestion be carried out in such a way that, When it is concluded, the contents of the digester including the liquor shall be blown out directly into a pulp-bin or blow-pit connected with the atmosphere, after which the liquor is extracted or separated from the pulp.

The invention is primarily characterized by the fact that the digestion acid, while it is being supplied into a digester for starting the cooking or digestion, is first heated in a first heating stage by indirect exchange of heat with hot Waste liquor or lye which has been ex tracted from the pulp-bin and subsequently heated further in a second heating stage by indirect exchange of heat with steam, cooled liquor which has been discharged from the first heating stage being stored and then introduced as a cooling medium or a cooling and flushing medium into the contents of a digester which have been rates Fatent G" ce 232m V Patented Feh. 25, 1 s

completely digested, or in the outlet of the pulp-bin when the contents of the digester are being blown.

The invention will be described in greater detail below with reference to the accompanying drawing, which diagrammatically shows an example of a practical embodiment of the invention.

Two digesters 1 and 2, in which cooking or digestion is carried on with adjustment of their mutual time ratios, can be blown separately through discharge valves 3 and 4 respectively, and a blow-pipe to a pulp-bin 6, which is connected with the atmosphere by means of an outlet 7. The waste lye or liquor from the pulp-bin can be led to a hot lye storage tank 10 through a connecting pipe 9 fitted with a pump. From the hot lye storage tank 10 hot lye can be caused by a pump 11 to pass through a pipe-line system 12, 13 through a heat exchanger 14 in which the lye is cooled so that it in a cool state enters a cold lye storage tank 15, from which cold lye can be passed by means of a pump 16 to either of the digesters 1 and 2 through a pipe line 17 and'the valves 18 and 19 respectively in the discharge throat of the respective digesters.

Digestion acid from the acid preparation can, through a valve 25), a pipe 21, a pump 22 and a pipe-line 23, be passed through the heat exchanger 14, in which the digestion acid is heated in indirect heat exchange relation to the waste lye passing through the same heat exchanger. The digestion acid thus heated is further conducted through a second heat exchanger 24, in which the digestion acid is further heated in an indirect heat exchange with steam to an appropriate temperature for feeding the digesters. The steam is led into this heat exchanger by a pipe-line 25, and the condensate of steam developed during the heat exchange is withdrawn from the heat exchanger by a tube 26, for example to a discharge pipe for the condensate, not shown in the drawing. Pipe-line 25 is fitted with a control valve 27, which, actuated by a temperature impulse in the digestion acid outlet from heat exchanger 24, acts to choke the how when the temperature there tends to exceed the appropriate value for feeding the digestion acid to the digester, and to open the flow when the temperature tends to fall below that value. By doing so the flow of steam is controlled so that the digestion acid with the intended constant, final temperature can be passed on through a control valve 2% and a conduit 3i) into either of the di esters 1 and 2 through valves 31 and 32 respectively. The control valve is of the overflow type, adapted to maintain a constant pressure on the flow side of the valve, that is to say in the heating system, said pressure being sufiiciently high to prevent gases from being liberated from the digestion acid while this is being heated. The tendency of the digestion acid to form incrustations in the heat exchangers is hereby strongly reduced. A shunt pipe 33 with a control valve 34 of the same kind as control valve 23, serves as a return line for the surplus of digestion acid carried through pump 22 when the instantaneous need of digestion acid for the digester is comparatively small in charging up stage so that a circulation of digestion acid can be maintained throughout the heating system. Control valve 34 is for this reason adjusted so as to open at a somewhat higher overpressure than control valve 28. In this way it is possible to maintain, irrespective of the instantaneous charging intensity, a constant high rate for the passage of the digestion acid through the heat exchangers which is favourable for the transmission of heat, and in order for reducing a possible tendency to incrustation.

The arrangement or" a preparatory heating up of the digestion acid with hot waste lye ensures a marked re- 7 duction in the peak loads in the boiler-house which are unavoidable inintermittent heating up with steam, as

in the present instance. The method of combining the heating up 'by steam with heating by means of waste lye also offers other irnportantadvanta'ges, Cooling of the digester content before 'this isblown to, the bin, so as to condense liberated vapors and retaining gases solved in the lye,'has, asknown in itself, for. example the advantage that gases which are disagreeable for the neighbourhood, can be practically prevented from escaping,

and' inaddition the bin can be made smaller than if it has to receive large quantitiesv of vapors and gases/ According to the invention, these advantagesj are moreover obtained'simultaneously as heat, which is released from. the contents of the digester on cooling and cong veying to the cooled waste lye, is recovered with the charged digestion acid which absorbs suchfheat fro the waste lye in its firstrstage of heating.

As the heat exchangers have to be utilized only durliquor, withdrawing the hot wia'st'e liquor from the blow.

v pit and storing, said waste liquor substantially at: its blow pit temperature, passing, in indire ct heat exchange relaa tion to each other, digestion acid which is being; charged to a digester and stored hot waste liquor,,so. as in a first stage to heat the digestion acid and to coolthe liquor,

storing the cooled waste liquor separately from. the hot i 7 waste liquor, passing the heated digestion acid and steam in indirect heatzexchange relation to each other, so as to increase inasecond stagethe'temperature of the diges- 'tion acid to the desired preheating'temperature, passing peratureand utilizing stored cooled spentlliquor asflush ing the period when'some digester; is charged with digestion acid, it is possible to arrange *for intervals when the heat exchangers can be cleaned without interrupting operations; When valve 20. has been closed, a cleansing liquid can then be taken in by pipe 21 through'a pipe 1 29 with a valve 35, pumped through the heat exchangers and discharged through a tube 36 having a valve 37. If desired, a cleansing liquid can-also be kept in circula- 7 tion through shunt line 33. A surplus of cooled waste lye obtained in the ordinary running of the plant can be withdrawn from the system through valve 38. V I V Thepractical example given here can of course be I modified in 'many ways within the scope of the invention by anyone skilled in the art. Valves have, for example,

only been shown inso far as absolutely necessary in order to illustrate the operation.

What I claim is: V V 1 In a method of digesting or cooking sulphite pulp,

the'digestion acid to the digester at said preheating teming and cooling agent in a digester blowing process.

2. A method according to claim '1, inwhich. the 'digestion; acid is I heated at a pressure which is hi'ghcnough tov prevent gases froml being liberated from the -diges+ tion acid during any part of the'acid heating'stages, i

3 .A method according to claim 1, in which digestion acid is passed through both acid heating- ;stages; in a larger-quantityper time unit than the quantity of acid instantaneously required for thecharging of the digester whereby, the superfluous quantity'of digestion a id l '7 ing the last heating stageis recirculated to the first heat- 'ing stage: 6 a v References Cited in the, filelofthispatent, V UNITED STATES: PATENTS 953,076

Waite t Mari 29, 1910 1,560,881 Ulmen. N6v..-10, 1925 1,691,682 Thorne ,No v..13, 1928 

1. IN A METHOD OF DIGESTING OR COOKING SULPHITE PULP, THE PROCESS OF PREHEATING THE DIGESTION ACID WHEN CHARGING A DIGESTER AND RECOVERING HEAT FROM WASTE LIQUOR COPMPRISING IN COMBINATION THE STEPS OF BLOWING A DIGESTER TO AN ATMOSPHERIC PRESSURE BLOW PIT, WHILE FLUSHING AND COOLING THE DIGESTER CONTENT WITH LOW TEMPERATURE WASTE LIQUOR, WITHDRAWING THE HOT WASTE LIQUOR FROM THE BLOW PIT AND STORING SAID WASTE LIQUOR SUBSTANTIALLY AT ITS BLOW PIT TEMPERATURE, PASSING, IN INDIRECT HEAT EXCHANGE RELATION TO EACH OTHER, DIGESTION ACID WHICH IS BEING CHARGED TO A DIGESTER AND STORED HOT WASTE LIQUOR, SO AS IN A FIRST STAGE TO HEAT THE DIGESTION ACID AND TO COOL THE LIQUOR, STORING THE COOLED WASTE LIQUOR SEPARATELY FROM THE HOT WASTE LIQUOR, PASSING THE HEATED DIGESTION ACID AND STREAM IN INDIRECT HEAT EXCHANGER RELATION TO EACH OTHER, SO AS TO INCREASE IN A SECOND STAGE THE TEMPERATURE OF THE DIGESTION ACID TO THE DESIRED PREHEATING TEMPERATURE, PASSING THE DIGESTION ACID TO THE DIGESTER AT SAID PREHEATING TEMPERATURE AND UTILIZING STORED COOLED SPENT LIQUOR AS FLUSHING AND COOLING AGENT IN A DIGESTOR BLOWING PROCESS. 